Cathodal trans-cranial Direct Current Stimulation (tDCS) modifies discrimination thresholds of the slope of the amplitude spectrum.

When asked to determine perceived contrast in natural scenes, humans rely more heavily on a subset of spatial frequency bands around the peak of the CSF (Haun & Peli, 2013). Interestingly, the bands used by observers shift as a function of the global amplitude spectra of natural images: observers use lower spatial frequencies for natural images with steeper slopes (α>1), and higher spatial frequencies for natural images with shallower slopes (α< 1). We aimed to explore this effect further by directly manipulating slope discrimination thresholds to both steep and shallow slopes with trans-cranial Direct Current Stimulation (tDCS). tDCS is a neuro-stimulator that modulates the membrane potential of cortical neurons and alters their responses to visually presented stimuli. The effects of tDCS are spatial frequency dependent, and only modulate contrast perception to high spatial frequencies. Thus, if observers rely more heavily on contrast at high spatial frequencies when the amplitude spectrum slope of an image is shallow, cathodal-tDCS (c-tDCS) should decrease discrimination thresholds by increasing contrast sensitivity to high spatial frequencies. Conversely, anodal-tDCS (a-tDCS) should increase discrimination thresholds for shallow slopes. Discrimination thresholds to steeper slopes should remain unaffected by tDCS. Participants completed two stimulation sessions, and began with either a-tDCS or c-tDCS, while a third group received sham both sessions. The c-tDCS first group showed a decrease in discrimination threshold for shallow slopes (α = 0.4, 0.7), and interestingly, also showed an increase in discrimination thresholds for steeper slopes (α = 1, 1.3, 1.6). An increase in contrast sensitivity to high spatial frequencies may benefit slope discrimination for shallow slopes, and furthermore, may also impede slope discrimination for steeper slopes. Our results are concordant with a mechanism, biased in spatial frequency, which encodes contrast in natural scenes in accordance to the relative responses of spatial frequency channels. Meeting abstract presented at VSS 2015.